For testing the proper operation of the modems, they are known to be looped back. With respect to this subject, reference be made to patent application Ser. No. 2,631,194 filed in France on 6 May 1988.
One of the problems that occurs during these loop-back operations is due to the independence of the transmitting and receiving clocks. Indeed, it is essential that the loop-back does not cause loop-back data to be lost. They may even be lost for a phase jump of less than T.sub.2 (T being the period of the clock signals) which occurs when these loop-back data are relayed. This loss appears of necessity for minor modulations of the type recommended by the V0.34 standard and leads to resynchronizations. At a high rate, according to this standard, a phase jump of only several degrees may cause a resynchronization to occur.